Mitigating the stimulated Brillouin scattering (SBS) problem in single-mode optical fibers is often based on complex doping allowing to create and spread multiple Brillouin resonances over a wide acoustic frequency range lowering an overall SBS gain maximum. Co-doping with Al2O3 is often used for it. However, it creates the "antiguiding" conditions for, at least, some of the acoustic waves electrostrictively excited in a vicinity of the fiber core. Conventional eigen-mode solvers have a fundamental restriction on guided acoustic modes and, thus, cannot allow one to calculate the SBS gain spectrum properly in a frequency region near and beyond the Brillouin frequency of pure silica. It requires the use of special approaches including an explicit account on the driving optical field. Here we provide a rigorous and, at the same time, relatively simple and fast method based on a development of the well-known infinite power-series expansion of a solution to the scalar inhomogeneous ordinary differential equation of the electrostrictively excited acoustic field, which has no requirement of specially fitting parameters and allows one to model exactly SBS gain spectra for arbitrarily doped (including different dopant species) conventional optical fibers. To complete the calculation framework, we also provide an experimental investigation of all required acousto-optical material parameters for GeO2, P2O5, Al2O3 and F dopants in MCVD-made fibers, on a base of which an Al2O3/GeO2/P2O5/F doping design of a single-mode optical fiber at 1.55 mu m was proposed and experimentally verified featuring a multi-resonant BGS providing similar to 11 dB of the SBS gain suppression relative to a standard single-mode germanosilicate fiber.

The minimum refractive index of the aluminophosphosilicate (APS) core in optical fibers has been determined for a wide range of phosphorous and aluminum concentrations. It was found that the APS core refractive index became higher by ~0.0005–0.0012 as compared to that in optical fiber preform. The analysis of the measured data has shown that at least 0.2 mol.% of Al2O3 and P2O5 remain in their ordinary form near their equimolar concentrations and do not form an AlPO4 join (the effect observed for all concentrations of AlPO4 join from 5 to 25 mol.%).

It is shown that even beam quality M-2 similar to 1.1 does not guarantee stability of the output radiation in large mode area fibers. This problem is especially crucial for narrow-linewidth amplifiers, where the output beam may exhibit variation of the shape and central position due to the presence of a small portion of higher-order modes. Spatially resolved spectral interferometry was used to determine the sources of higher-mode excitation in a tapered fiber amplifier. It is shown that implementation of tapered fibers with an additional cladding mode stripper at the thin tapered fiber end allows achievement of an ultimately low level of higher-order modes content (less than 0.1 %). A perfectly stable output beam was demonstrated up to an average power of at least 50 W.

Recent state-of-the-art for ultra-highly Yb-doped silica-based fibers have been presented. Detailed properties of the developed fibers, i.e. refractivity of different co-dopants, background loss behavior due to Yb-doping, photodarkening effects and lasing efficiency degradation at high Yb concentrations have been studied. Application of highly Yb-doped fibers for obtaining of lasers and amplifiers of an extremely shot lengths have been discussed. Novel results regarding development of ultra-highly Yb-doped large-mode-area (LMA) fiber amplifier have been presented.

In this study, we explored the potential for average power scaling in a monolithic side-counter-pumped combiner based on Yb-doped tapered fibers. The optimal configuration of the pump-feeding fibers was determined through experiments with passive signal fibers. It is shown that pump coupling efficiencies higher than 83% can be achieved for fibers coated with low-index polymer with a numerical aperture (NA) around 0.45 and more than 74% for fibers with second cladding made of F-doped silica (NA ~ 0.26) for pump power up to 100 W. It was shown that the main factor significantly reducing the pump-to-signal conversion efficiency in the developed monolithic Yb-doped tapered fiber amplifiers is the pump leakage due to the decrease of the first cladding diameter along the tapered fiber and the corresponding increase of the pump NA (which becomes higher than the NA of the first cladding). A solution to this problem based on a narrowing diameter at the output end of the tapered fiber was proposed and realized. The record-high average power of 41 W, with a coupling efficiency of 77.7%, was demonstrated in a monolithic amplifier with a threshold of nonlinear effects of more than 600 kW (for ps pulses). Prospects for further power scaling in all-fiber sub-MW peak power amplifiers are discussed.

A novel Yb-doped fiber design for improved lasing near 976 nm based on spectral filtering of the amplified spontaneous emission near 1030 nm was realized and investigated. A very sharp short-pass filter was implemented by adding appropriately chosen high-index absorbing rods into the silica cladding. In this case, the resonant interaction of the core mode with the high-index rod mode could be controlled by fiber bending, which allows for the precise adjustment of the stop-band position. It was shown that the utilization of Sm-doped absorbing rods allows one to achieve very high absorption of emission at unwanted wavelengths, but it also adds background losses for the pump near 915 nm and for the signal at 976 nm. Despite this fact, the improvement of efficiency in the 976 nm fiber amplifier, after shifting the stop-band to 1000 nm, was clearly demonstrated. Based on theoretical calculations, it was shown that, after optimizing the fiber parameters, a further twofold improvement in efficiency was possible despite the excess losses at the pump and signal wavelengths.

A newly-designed Yb-doped fiber type, intended to enhance 976 nm lasing by suppressing the spectral component including 1030 nm amplified spontaneous emission (ASE), was thoroughly researched. To implement an extremely sharp short-bandpass spectral filter, several suitably selected high-index absorber rods were incorporated into the fiber cladding. Thus, the concept of such a filter allows for a relatively accurate stop-band position adjustment as a result of the highly fiber-bend-dependent state of the intermodes resonance core-rod interaction. Finally, the impact of enhancing the power conversion efficiency (PCE) of the fiber amplifier emitting at a wavelength of 976 nm, by shifting the stop-band, is justified and demonstrated in this study.

We demonstrated an optimization of a picosecond fiber amplifier based on Yb-doped tapered fiber in a spectral range of 1030 nm. Nonlinear effects limiting peak power scaling (stimulated Raman scattering and four-wave mixing) were studied and factors affecting their threshold were established, such as gain, diameter profile along the length of taper, output mode field diameter, and numerical aperture of a pump. By determining the optimal amplification regime and manufacturing advanced tapered fibers, we amplified 13 ps pulses to a record-high peak power of 1 MW at a wavelength of 1029 nm directly at the output of the fiber at an average power of 13.8 W. Four-wave mixing was the limiting factor, and the total fraction of deleterious components in the output spectrum was ~2%. The quality of the output beam was close to being diffraction limited (M2 < 1.2).

High peak and average power Yb-doped ps-pulse fiber amplifiers are of high demand for different micromachining tools. Most of commercial lasers are designed for operation near 1030 nm, which is due to high gain at this wavelength. An ultimately high peak power of ∼ MW just after the fiber amplifier and ∼3.8 GW after pulse compression [1] was achieved in lasers operated near 1030 nm by utilization of rod-type photonic crystal fibers (PCF). However, such fibers have a well-known problem: they could not be spliced with standard fibers, and also it must be kept perfectly straight. As a result, lasers based on PCF lose most of the advantages of fiber lasers – reliability, compactness and a low production cost.

In this article, we studied high-energy single frequency transform-limited Er-doped amplifiers core pumped by a specially developed Yb-doped fiber laser at 980 nm and a Raman laser at 1480 nm. It was demonstrated that pumping at 1480 nm allows achieving slightly higher maximum pulse energy for long pulses in the absence of nonlinear effects. At the same time, amplifiers with a pump at 980 nm have a higher threshold of stimulated Brillouin scattering (SBS), and higher peak power of shorter pulses can be achieved in such a scheme. Lasers with more than 730 μJ pulse energy and with higher than 3.5 kW peak power were demonstrated.

A significant change in the refractive index profiles for the large mode area phosphoroaluminosilicate (PAS) core optical fibers was observed in comparison to that in preforms. This study shows that the refractive index of the PAS core can vary from negative (in preform) to positive (in fiber), and the difference in the refractive index between the core and preform can exceed a few thousand. By measuring a large set of fibers with different concentrations of P2O5 and Al2O3, we define the refractivity of each dopant (P2O5, Al2O3 and AlPO4 joint) after drawing fiber from the preform and discuss the possible origin of the observed refractive index variation.

Silica-based optical fibers with an ultra-high Yb concentration were systematically studied. Three the most commonly used in industry glass matrixes for active fiber core were investigated: aluminosilicate, phosphosilicate and aluminophosphosilicate. For all the glass hosts optical fibers doped with a record high concentration of Yb in a glass core were fabricated utilizing an all-gas-phase deposition based on MCVD technology. The factors limiting increase of Yb content in glasses and fibers were revealed. For the first time it was shown that highly Yb-doped fibers could nearly completely lose their active properties and the most probable reason for that is concentration quenching of luminescence.

In the present work we have developed and realized a high core-to-cladding diameters ratio active optical fibers operated in a single-mode regime due to specially designed structure containing boron-doped and fluorine-doped rods. The beam quality at the output of the realized fibers was studied by three independent methods: near field investigation, M-2 technique and S-2 technique. According to the obtained results, the developed approach allows efficient suppression high-order-modes (first of all-LP11 mode) in the core and achieves a diffraction-limited beam at the output of the appropriately bent fiber (suppression of unwanted modes was better than 30 dB). Additionally, it is shown that application of such approach allows for increased bent-resistance of the fundamental mode and realizes polarization-sensitive amplification.

A tapered Er-doped fiber amplifier for high peak power pulses amplification has been developed and tested. The core diameter changed from 15.8 µm (mode field diameter (MFD) 14.5 µm) to 93 µm (MFD 40 µm) along 3.7 m maintaining single-mode performance at 1555 nm (according to the S2-method, the part of the power of high-order modes does not exceed 1.5%). The amplification of 0.9 ns pulses with spectral width below 0.04 nm up to a peak power above 200 kW (limited by self-phase modulation) with a slope pump-to-signal conversion efficiency of 15.6% was demonstrated.

We demonstrated a simple design of a monolithic all-fiber side-coupled combiner for counter-pumped amplifiers that requires no special fiber processing systems for fabrication. The combiner based on a Yb-doped polarization-maintained tapered fiber with an output core diameter of 40 µm and a total length of 1.8 m exhibiting over 60% coupling efficiency of 976 nm 0.10 NA pump power was demonstrated and utilized to amplify 1064 nm 9.3 ps 1.84 MHz pulses up to 9.1 W of average power and 0.53 MW of peak power with near diffraction-limited beam quality. The demonstrated approach seems promising for further power scaling, retaining good output beam characteristics via design optimization.

In the present work we have proposed design of an active large-mode-area high core-to-cladding diameters ratio polarizing quasi-Bragg fiber.

We demonstrated an all-fiber chirped-pulse amplifier with a stretcher stage based on a triple-cladding fiber and a final amplification stage based on a newly-developed highly Yb-doped large-mode area fiber with a Ge-doped pedestal. The stretcher fiber was designed to match a diffraction grating compressor in the third-order dispersion and allowed the stretching of 6 ps 1026 nm chirped pulses up to ~500 ps. These pulses were amplified in the developed Yb-doped fiber and then compressed using a transmission grating compressor down to 670 fs duration with 3.5 W average power and $61.8~\mu \text{J}$ pulse energy, corresponding to a record-high peak power of 92 MW.

Spectra of the initial loss and radiation-induced attenuation are compared in pure-silica-core optical fibers (PSFs) drawn from the same preform with the help of carbon-resistance heaters (CRH) or an oxyhydrogen flame torch (OFT). OFT-based drawing was found to increase the initial loss at lambda = 1.31 and 1.55 mu m by similar to 2-3 dB/km owing to growth of the 1.38-mu m OH-group and 1.53-mu m SiH-group absorption bands. RIA was essentially reduced in the OFT-drawn PSF, because of suppression of self-trapped holes. Therefore, OFT-drawing of radiation-resistant PSF is argued to be promising, provided a tradeoff between the initial loss and RIA is found.

The possibility to scale-up output pulse energy in diffraction-limited Er-doped fiber amplifier has been studied. It is shown that the utilization of tapered fiber design allows one to increase the pulse energy up to 2 mJ, while keeping the diffraction-limited beam quality (M2~1.4). Factors limiting the further increase in pulse energy are revealed.